Transmission lines may carry foreign electrical energy from, for example, transient voltage surges which can be generated by lightning strikes, industrial equipment, or the utilities themselves, as well as other equipment within a communications system. In order to safeguard electronic equipment, such as telecommunications equipment, computers, and instrumentation equipment, connected to such lines, traditional protective methods have included the use of gas discharge tubes, carbon blocks, fuses, heat coils, MOVs and other types of devices and wiring schemes. These methods do not provide a sufficient level of protection required for solid state circuits used in present day electronic equipment. These prior methods are slow to respond, allow significant voltage spikes above the clamping voltage before they begin to conduct, and respond with voltage overshoot.
Protection circuits have also been developed which include semiconductor elements, such as the protection circuit described and claimed in U.S. Pat. No. 4,408,248. However, such circuits have not proven satisfactory because they have required numerous components which are costly and which require excessive physical space. Other protection circuits have used a single solid state switch formed of N-type silicon material, such as RCA's Surjector, where once the breakover voltage is exceeded, there is a positive slope on the current-voltage diagram. Such a protection circuit dissipates additional energy until it turns on. The voltage overshoot is typically 5 to 15 volts or more, depending on the speed of the transient that the device is trying to protect.
A need has thus arisen for an overvoltage protection circuit that has a fast reaction time, offers tight voltage control, and provides for a lower power dissipation during switching. Additionally, a need has arisen for an overvoltage protection circuit that is economical, and which has reduced component requirements and reduced physical size, while maintaining high surge current capacities.